The present invention relates to a method of isotope separation utilizing thermal diffusion and a thermal diffusion column used for the practice thereof, and in particular to a method of isotope separation utilizing thermal diffusion which can improve the separation efficiency by reducing undesirable isotopic equilibrium reactions, and also to a thermal diffusion column used for the practice thereof.
In the areas of nuclear engineering, life science and others the use of isotopes is going to increase. Isotopes are produced (separated) mainly by the methods of ion exchange, membrane and the like.
Besides, isotopes can be roughly separated by the low temperature distillation and other methods, and then can be enriched by the thermal diffusion process. It is desired to improve the efficiency of isotope separation by these methods. Hereinafter an explanation will be made in regard to the isotope separation which utilizes thermal diffusion.
So far the isotope separation utilizing thermal diffusion has been carried out by a thermal diffusion column equipped with a hot wire along the central axis. The outer wall of thermal diffusion column is covered with a water cooled jacket in order to prevent rise of temperature. In such a thermal diffusion column, if the central hot wire is heated to a temperature between 900 K. and 1300 K., a large temperature gradient along the radial direction is formed.
The mean kinetic energy of a gaseous molecule E.sub.K is expressed as follows. EQU E.sub.K =(1/2)mv.sup.2
For example, in the case of a gas mixture comprising hydrogen molecules H.sub.2 and deuterium molecules D.sub.2, the mass of a gaseous molecule is 2 for H.sub.2 and 4 for D.sub.2 respectively, and therefore there is a great difference in the mean velocity at the same temperature between H.sub.2 molecules and D.sub.2 molecules.
As described above, if a large temperature gradient is formed in a thermal diffusion column and thereinto a gas mixture of (H.sub.2 +D.sub.2) is introduced, the gaseous molecules having different masses will respectively make different distributions along the temperature gradient, and thus the molecules having larger masses are enriched in the side of lower temperature and the molecules having smaller masses are enriched in the side of higher temperature.
Further, a convection occurs in the thermal diffusion column, so that the gas of higher temperature around the hot wire goes up and the gas of lower temperature in the neighborhood of the water cooled wall goes down. Thus, a gas enriched with D.sub.2 is obtained from the bottom of thermal diffusion column and a gas enriched with H.sub.2 is obtained from the top of thermal diffusion column.
For example, in a case of separating a gas mixture comprising two hydrogen isotopes with a composition of H.sub.2 :D.sub.2 =50 vol. %:50 vol % (hereinafter referred to simply as %), a thermal diffusion column vertically set up having an inside diameter of about 30 mm and a length of about 92 cm was used and operated with a temperature of hot wire at about 1273 K. and a gaseous pressure of about 700 Torr, and then the concentration of D.sub.2 at the bottom of thermal diffusion column attained to 56%. According to the provided condition, the attained concentration of D.sub.2 varies to a certain extent.
Among the hydrogen isotopes beside deuterium (D), there is tritium (T). In the case of a gas mixture of three components (H.sub.2 +D.sub.2 +T.sub.2), it is possible to separate heavy molecules from light molecules according to the same principle as mentioned above.
In the isotope separation from a gas mixture of (H.sub.2 +D.sub.2) by means of such a thermal diffusion column as mentioned above, simultaneously with the thermal diffusion there proceeds an isotopic equilibrium reaction as follows. EQU H.sub.2 +D.sub.2 .fwdarw.2HD
That is to say, in the same manner as typical chemical reactions, the above reaction proceeds and produces HD in a definite equilibrium concentration. In the conventional thermal diffusion columns, the surface of hot wire which typically is make of tungsten (W), is heated to a high temperature between 900 K. and 1300 K., and in addition the surface of tungsten wire has a catalytic activity at higher temperatures, so that in a short time the isotopic equilibrium reaction proceeds to produce gaseous HD. In the result, a gas mixture of two components is converted into that of three components, and thus the degree of separation is extremely reduced. Besides, instead of the tungsten hot wire, also a stainless steel wire can be used. The latter has a higher catalytic activity than the former and can lower the hot wire temperature in the extent of about 100.degree. C. under that of the former.
As mentioned above, in the conventional isotope separation utilizing thermal diffusion, the undesirable isotopic equilibrium reaction proceeds at the same time, which has caused the reduction of separation efficiency.